Fable is one of the hardest games to rate or review because it got so much hype before its release. In the end, Fable is a fun action/RPG that does lots of things right, but most of the features come off as gimmicks.
You can do lots of things in Fable that you can't do in other games, but most of them aren't that interesting. For a detailed list of what you can and can't do, I recommend the IGN review.
The problem is that none of those things you can do are really that interesting, or have much of an affect on gameplay. Most of your time will be spent doing the same things you do in other action/RPGs, like fetch quests, protection quests, and experience farming. Sure, you can change your name, and people will call you by that name, but in the end it's just not that fun.
Ironically, while the gimmicks don't really do it for me, the action/RPG setup is great. The experience system feels very rewarding, combat isn't bad at all, and the game is fun. It's far from the world-beater it was supposed to be, but it's a good (though short) time.
Fable plays a lot like The Legend of Zelda: Ocarina of Time, except it's heavier on the RPG elements, and the puzzles aren't nearly as clever. It has more distractions, but in the end that's all they are.